1. Field of the Invention
The present invention relates to an optical scanning device, and more particularly to an optical scanning device that is suited to be employed in an electrophotographic image forming apparatus such as an electrophotographic printer or copying machine.
2. Description of Related Art
In recent years, for this kind of optical scanning devices, using a VCSEL (vertical cavity surface emitting laser) having a multiple of light emitting points in a light source has been suggested so as to comply with demands for high-density image formation and for high-speed image formation.
Japanese Patent Laid-Open Publication No. 2003-182139 suggests that streaky density unevenness due to reciprocity failure be reduced by setting the light quantity around the boundary of a scanning area to a low value. Japanese Patent Laid-Open Publication No. 2004-77714 suggests that density unevenness around the boundary be made inconspicuous by performing interlace scanning wherein a plurality of beams are scanned on scanning lines at regular intervals. Japanese Patent Laid-Open Publication No. 2007-168299 suggests that light emitting points be arranged in a thinned way such that overlap portions subjected to a multiple of scanning motions will not be thicker.
In the art of setting the light quantity around the boundary to a low value, however, there causes the following problem. When an image including both a pattern to be formed by a multiple of scanning motions and a pattern to be formed by one scanning motion is formed, the density of the latter pattern will be low because the boundary, where the light quantity is low, is not an overlap portion. In the art of performing interlace scanning, also, there causes the following problem. In interlace scanning, a larger area is scanned at the same time, and the scanning is susceptible to errors made temporarily, such as driving unevenness of the photosensitive drum.
In the art of thinning light emitting points, there causes the following problem. Although the number of light emitting points is small, light emitting points in the peripheral part are located far from the optical axis, and the light emitting points in the peripheral part and the light emitting points in the central part have a great difference in the optical performance. More specifically, in view of thermal crosstalk and the wiring from the light emitting points in the peripheral part to the light emitting points in the central part, there is a limit in close alignment of the light emitting points. Accordingly, it is necessary to arrange some light emitting points far away from the optical axis, and consequently, the formed image is influenced by the optical aberration.